Linkage for irreducible representations of ${\rm GL}_3$ in local Base change

Sabyasachi Dhar

Published 2023-10-31Version 1

Let $E$ be a finite Galois extension of a $p$-adic field $F$ with degree of extension $l$, where $l$ and $p$ are distinct primes with $p\ne 2$. Let $\pi_F$ be an integral cuspidal $\overline{\mathbb{Q}}_l$-representation of ${\rm GL}_3(F)$ and let $\pi_E$ be the base change lifting of $\pi_F$ to the group ${\rm GL}_3(E)$. Then for any ${\rm GL}_n(E)\rtimes{\rm Gal}(E/F)$-stable lattice $\mathcal{L}$ in the representation space of $\pi_E$, the Tate cohomology group $\widehat{H}^0({\rm Gal}(E/F),\mathcal{L})$ is defined as $\overline{\mathbb{F}}_l$-representation of ${\rm GL}_3(F)$, and its semisimplification is denoted by $\widehat{H}^0(\pi_E)$. In this article, we show that the Frobenius twist of mod-$l$ reduction of $\pi_F$ is isomorphic to the Tate cohomology group $\widehat{H}^0({\rm Gal}(E/F),\mathcal{L})$. This result is related to the notion of linkage under the local base change setting. Moreover, this notion of linkage under the local base change for non-cuspidal irreducible representations of ${\rm GL}_3$ is also investigated in this article.